Candidate: Medhat Samaan
Title: Fabrication of High-Performance Probes for Atomic Force Microscope (AFM)
Date: February 25, 2021
Time: 8:00 AM
Place: REMOTE ATTENDANCE
Supervisor(s): Cui, Bo
Abstract:
This thesis is focused on nanofabrication and its application in high aspect ratio (HAR) AFM tips as well as HAR nanopillars. Atomic force microscopy (AFM) images the level of the atom through a sharp tip that touches the substrate, but it can produce false images with tall and dense features due to the shape and dimensions of the tip. This problem is solved by achieving a high aspect ratio (HAR) AFM tip. HAR AFM tips are often fabricated individually involving expensive instruments.
We present a simple repeatable and low-cost to fabricate the edge AFM probes in batches, from the regular commercial probe. This is accomplished by using the Cr2O3 mask on one side of the tip then etch away the Si at the other side to release the apex position. We also, successfully introduced a method to fabricate the high aspect ratio AFM probes from regular commercial tips, in batches. The main procedure is to form a small Cr2O3 dot mask on the tip apex of each tip, by overlap Al at the top of Cr2O3, then etch the Si to reveal the HAR tips in the range of 15:1. This method is cost effective and with much higher throughput as it is done in a batch and lithography free. Because tip sharpening using oxidation is the most commercially used method, we studied the oxidation process and found that the regular commercial tip two steps oxidation of 950°C for half to one hour each, yields the best tip sharpening result of an apex diameter (between 2 to 5 nm).
Our investigation of the ICP RIE pseudo Bosch recipe which has a 400nm/min etch rate and, reveals that the etching rate can be increased to 500 nm/min by raising RF power from 20 W to 50 W and further increase the etching rate to 715 nm/min by raising the ICP power from 1200 W to 2000 W. Beside, the study of the new etching rate and profile for different C4F8/ SF6 gas ratios of 55/5, 50/10, 40/20, 30/30, 20/40, 10/50, 5/55 as well as especially investigate the cone shape and find that 50/10 is the optimum ratio for the cone shape pillars production. Also, sharpening the pillars array using wet etch employing HNO3 and HF with a ratio of 200:1 for 8 min, result in an aspect ratio of 135 at 10 µm high and diameter of 73 nm from ~755 nm.
Fabricate HAR is successfully achieved by converting a regular pyramidal shaped probe into a HAR probe and reducing FIB milling volume from 0.5 µm3 to around 0.01 µm3 and replacing it with RIE in batch style. Further successful work has been performed to automate the process using a programmable FIB.
We demonstrate the use of FIB patterns in both Ga and Au implanted as an etch mask. Using the Ga FIB with 8125 µC/cm2 is the optimum dose for Si as a mask and get nanopillar of 653 nm height with the base radius of 317 nm diameter. Also, 3219 µC/cm2 is the optimum dose for Au FIB implantation resulting in nanopillar of 880 nm high and 338 nm base diameter. This technique can be applied to a regular commercial tip to transform it into a HAR tip.